1. Field of the Invention
The present invention relates to a multiplexing method and equipment for the multicast communication (selectively simultaneous communication) in ATM(Asynchronous Transfer Mode) communication.
2. Description of the Prior Art
There is known a multicast connection as one of the techniques supporting the multicast communication in ATM communication. In the point to multi point connection, among them, ATM cells from different transmitting nodes toward one receiving node are multiplexed into one UNI (User Network Interface) when transmitted. In this case, it is necessary that, at the receiving node, the ATM cells with the same VPI/JVCI (Virtual Pass Identity/Virtual Channel Identity) addresses be identified for each receiving node and be re-combined into a packet of (ATM Adaptation Layer).
Hitherto, the above mentioned identification was implemented by by using MID (Multiplexing Identification) field of AAL3/4, as shown in ITU-T 1.363.
The ATM cell format of AAL3/4 is shown in FIG. 7. As shown in (a) of FIG. 7, the ATM cell of AAL3/4 has 10 bit MID field 71 for multiplexing. When the value of this field is set so as to be different for each receiving node, multiplexed ATM cells can be identified at the receiving node.
As shown in FIG. 7, comparing the ATM cell of AAL3/4 with the ATM cell of AAL5 as recommended by ITU-T, the payload 73 of ATM cell of AAL5 is 48 octet payload, while that of AAL3/4 is 44 octet and therefore, has greater overhead than AAL5. Accordingly, the communication by using the ATM cell of AAL3/4 is less efficient than AAL5. Furthermore, the ATM cell of AAL3/4 is costly and complex in management, because AAL3/4 has 5 functional fields including MID field 71 in addition to the cell header.
On the other hand, when more efficient AAL5, instead of AAL3/4, is employed for multiplexing, there is the disadvantage that the multiplexed cells per se can not be distinguished at receiving node, because AAL5 has not any identification field corresponding to the MID field of AAL3/4 for multiplexing.